*Observations on the Skin and its Appendages.*-Examine the palm of the hand with a lens. Note the small ridges which correspond to the rows of papillae beneath the cuticle. In these find small pits, which are the openings of the sweat glands.

2. Examine the epidermis on the back of the hand and palm. At which place is it thickest and most resisting? Is it of uniform thickness over the palm? Try picking it with a pin at the thickest place, noting if pain is felt. Inference?

3. Examine a finger nail. Is the free edge or the root the thickest? Trim closely the thumb nail and the nail of the middle finger of one hand and try to pick up a pin, or other minute object, from a smooth, hard surface.

The result indicates what use of the nails? Suggest other uses.

4. Examine with a microscope under a low power hairs from a variety of animals, as the horse, dog, cat, etc., noting peculiarities of form and surface.

*To ill.u.s.trate Cooling Effects of Evaporation.*-1. Wet the back of the hand and move it through the air to hasten evaporation. Observe that, as the hand dries, a sensation of cold is felt. Repeat the experiment, using ether, alcohol, or gasolene instead of the water, noting the differences in results. These liquids evaporate faster than water.

2. Wet the bulb of a thermometer with alcohol or water. Move it through the air to hasten evaporation. Note and account for the fall of the mercury.

CHAPTER XVII - STRUCTURE OF THE NERVOUS SYSTEM

*Coordination and Adjustment.*-If we consider for a moment the movements of the body, we cannot fail to note the cooperation of organs, one with another. In the simple act of whittling a stick one hand holds the stick and the other the knife, while the movements of each hand are such as to aid in the whittling process. Examples of cooperation are also found in the taking of food, in walking, and in the performance of different kinds of work. Not only is cooperation found among the external organs, but our study of the vital processes has shown that the principle of cooperation is carried out by the internal organs as well. The fact that all the activities of the body are directed toward a common purpose makes the cooperation of its parts a necessity. The term "coordination" is employed to express this cooperation, or working together, of the different parts of the body.

A further study of the movements of the body shows that many of them have particular reference to things outside of it. In going about one naturally avoids obstructions, and if anything is in the way he walks around or steps over it. Somewhat as a delicate instrument (the microscope for example) is altered or adjusted, in order to adapt it to its work, the parts of the body, and the body as a whole, have to be _adjusted_ to their surroundings. This is seen in the att.i.tude a.s.sumed in sitting and in standing, in the position of the hands for different kinds of work, in the variations of the circulation of the blood in the skin, and in the movements for protecting the body.(95)

*Work of the Nervous System.*-How are the different activities of the body controlled and coordinated? How is the body adjusted to its surroundings?

The answer is found in the study of the nervous system. Briefly speaking, the nervous system controls, coordinates, and adjusts the different parts of the body by fulfilling two conditions:

1. It provides a complete system of connections throughout the body, thereby bringing all parts into communication.

2. It supplies a means of controlling action (the so-called impulse) which it pa.s.ses along the nervous connections from one part of the body to another.

The present chapter deals with the first of these conditions; the chapter following, with the second.

*The Nerve Skeleton.*-If all the other tissues are removed, leaving only the nervous tissue, a complete skeleton outline of the body still remains.

This nerve skeleton, as it has been called, has the general form of the framework of bones, but differs from it greatly in the fineness of its structures and the extent to which it represents every portion of the body. An examination of a nerve skeleton, or a diagram of one (Fig. 125), shows the main structures of the nervous system and their connection with the different parts of the body.

Corresponding to the skull and the spinal column is a central nervous axis, made up of two parts, the _brain_ and the _spinal cord_. From this central axis white, cord-like bodies emerge and pa.s.s to different parts of the body. These are called _nerve trunks_, and the smaller branches into which they divide are called _nerves_. The nerves also undergo division until they terminate as fine thread-like structures in all parts of the body. The distribution of nerve terminations, however, is not uniform, as might be supposed, but the skin and important organs like the heart, stomach, and muscles are the more abundantly supplied. On many of the nerves are small rounded ma.s.ses, called _ganglia_, and from many of these small nerves also emerge. At certain places the nerves and ganglia are so numerous as to form a kind of network, known as a _plexus_.

[Fig. 125]

Fig. 125-*Diagram of nerve skeleton.* The ill.u.s.tration shows the extent and general arrangement of the nervous tissue. _A._ Brain. _B._ Spinal cord. _N._ Nerve trunks and nerves. _G._ Ganglia.

It is through these structures-brain and spinal cord, nerve trunks and nerves, ganglia and nerve terminations-that connections are established between all parts of the body, but more especially between the surface of the body and the organs within.

*The Neurons, or Nerve Cells.*-While a hasty examination of the nerve skeleton is sufficient to show the connection of the nervous system with all parts of the body, no amount of study of its gross structures reveals the nature of its connections or suggests its method of operation. Insight into the real nature of the nervous system is obtained only through a study of its minute structural elements. These, instead of being called cells, as in the case of the other tissues, are called _neurons_. The use of this term, instead of the simpler one of nerve cell, is the result of recent advances in our knowledge of the nervous system.(96)

[Fig. 126]

Fig. 126-*Diagram of a mon-axonic neuron* (greatly enlarged except as to length). The central thread in the axon is the axis cylinder.

The neurons are in all respects cells. They differ widely, however, from all the other cells of the body and are, in some respects, the most remarkable of all cells. They are characterized by minute extensions, or prolongations, which in some instances extend to great distances. Though the neurons in certain parts of the body differ greatly in form and size from those in other parts of the body, most of them may be included in one or the other of two cla.s.ses, known as _mon-axonic_ neurons and _di-axonic_ neurons.

*Mon-axonic Neurons.*-Neurons of this cla.s.s consist of three distinct parts, known as the cell-body, the dendrites, and the axon (Fig. 126).

The _cell-body_ has in itself the form of a complete cell and was at one time so described. It consists of a rounded ma.s.s of protoplasm, containing a well-defined nucleus. The protoplasm is similar to that of other cells, but is characterized by the presence of many small granules and has a slightly grayish color.

The _dendrites_ are short extensions from the cell-body. They branch somewhat as the roots of a tree and form in many instances a complex network of tiny rootlets. Their protoplasm, like that of the cell-body, is more or less granular. The dendrites increase greatly the surface of the cell-body, to which they are related in function.

The _axon_, or nerve fiber, is a long, slender extension from the cell-body, which connects with some organ or tissue. It was at one time described as a distinct nervous element, but later study has shown it to be an outgrowth from the cell-body. The mon-axonic neurons are so called from their having but a single axon.

*Di-axonic Neurons.*-Neurons belonging to this cla.s.s have each a well-defined cell-body and two axons, but no parts just like the dendrites of mon-axonic neurons. The cell-body is smooth and rounded, and its axons extend from it in opposite directions (Fig. 127).

[Fig. 127]

Fig. 127-*Diagram of a di-axonic neuron.* The diagram shows only the conducting portion of the axon, or axis cylinder.

*Structure of the Axon.*-The axon, or nerve fiber, has practically the same structure in both cla.s.ses of neurons, being composed in most cases of three distinct parts. In the center, and running the entire length of the axon, is a thread-like body, called the _axis cylinder_ (Fig. 126). The axis cylinder is present in all axons and is the part essential to their work. It may be considered as an extension of the protoplasm from the cell-body. Surrounding the axis cylinder is a thick, whitish-looking layer, known as the _medullary sheath_, and around this is a thin covering, called the _primitive sheath_, or neurilemma. The medullary sheath and the primitive sheath are not, strictly speaking, parts of the nerve cell, but appear to be growths that have formed around it. Certain of the axons have no primitive sheath and others are without a medullary sheath.(97)

*Form and Length of Axons.*-Where the axons terminate they usually separate into a number of small divisions, thereby increasing the number of their connections. Certain axons are also observed to give off branches before the place of termination is reached (Fig. 131). These collateral branches, by distributing themselves in a manner similar to the main fiber, greatly extend the influence of a single neuron.

In the matter of length, great variation is found among the axons in different parts of the body. In certain parts of the brain, for example, are fibers not more than one one-hundredth of an inch in length, while the axons that pa.s.s all the way from the spinal cord to the toes have a length of more than three feet. Between these extremes practically all variations in length are found.

*Arrangements of the Neurons.*-Nowhere in the body do the neurons exist singly, but they are everywhere connected with each other to form the different structures observed in the nerve skeleton. Two general plans of connection are to be observed, known as the anatomical and the physiological, or, more simply speaking, as the "side-by-side" and "end-to-end" plans. The side-by-side plan is seen in that disposition of the neurons which enables them to form the nerves and the ganglia, as well as the brain and spinal cord. The end-to-end connections are necessary to the work which the neurons do.

*Side-by-side Connections.*-On separating the ganglia and nerves into their finest divisions, it is found that the nerves consist of axons, while the ganglia are made up mainly of cell-bodies and dendrites. The axons lie side by side in the nerve, being surrounded by the same protective coverings, while the cell-bodies form a rounded ma.s.s or cl.u.s.ter, which is the ganglion (Fig. 128). But the axons, in order to connect with the cell-bodies, must terminate within the ganglion, so that they too form a part of it. To some extent, also, axons pa.s.s through ganglia with which they make no connection. The neurons in the brain and spinal cord also lie side by side, but their arrangement is more complex than that in the nerves and ganglia.

[Fig. 128]

Fig. 128-*Diagrams ill.u.s.trating arrangement of neurons.* _A, B._ Ganglia and short segments of nerves. 1. Ganglion. 2. Nerve. In the ganglion of _A_ are end-to-end connections of different neurons; in the ganglion of _B_ are the cell-bodies of di-axonic neurons. _C._ Section of a nerve trunk. 1. Epineurium consisting chiefly of connective tissue. 2. Bundles of nerve fibers. 3. Covering of fiber bundle, or perineurium. 4. Small artery and vein.

The side-by-side arrangement of the neurons shows clearly the structure of the ganglia and nerves. The nerve is seen to be a bundle of axons, or nerve fibers, held together by connective tissue, while the ganglion is little more than a cl.u.s.ter of cell-bodies. Their connection is necessarily very close, for the same group of neurons will form, with their axons, the nerve, and, with their cell-bodies, the ganglion (Fig. 128).

*End-to-end Connections.*-These consist of loose end-to-end unions of the fiber branches of certain neurons with the dendrites of other neurons. The purpose of such connections is to provide the means of communication between different parts of the body. There appears to be no actual uniting of the fiber branches with the dendrites, but they come into relations sufficiently close to establish _conduction pathways_, and these extend throughout the body (Fig. 129). They connect all parts of the body with the brain and spinal cord, while connections within the brain and cord bring the parts into communication with each other.

[Fig. 129]

Fig. 129-*Diagram of a nerve path* starting at the skin, extending through the spinal cord, and pa.s.sing out to muscles. A division of this path also reaches the brain.

*Nature of the Nervous System.*-The nervous system represents the sum total of the neurons in the body. In some respects it may be compared to the modern telephone system. The neurons, like the electric wires, connect different places with a central station (the brain and spinal cord), and through the central station connections are established between the different places in the system. As the separate wires are ma.s.sed together to form cables, the neurons are ma.s.sed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pa.s.s to a study of the gross structures observed in the nerve skeleton.